1. Field of Invention
This invention relates to a gyrocompass and to a new and improved technique and circuit for improving the settling time of a gyrocompass while maintaining its accuracy.
More particularly, the invention relates to a new and improved gyro compass employing a two-axis gyroscope and novel decoupling compensation circuit intercoupling the outputs of the two axes for eliminating the inherent cross coupling between the two axes to thereby improve the settling time of the gyrocompass without adversely affecting its accuracy.
2. Description of the Prior Art
Gyrocompasses have a fundamental trade off between settling time and alignment error due to erection error. This trade off becomes evident from a detailed analysis of the manner of operation of a gyrocompass. A common type of gyrocompass operates by sensing the rotation of the earth and aligning a gyro spin axis so that the component of the earth's rotation sensed by the gyro about an input axis is zero. It is the function of the erection system to provide a rigid coupling to the earth so that when the rate sensed by the gyro has be nulled, the gyro input axis is truly orthogonal to the earth's spin axis. If the erection system (i.e., the stabilized platform surface maintained parallel to the earth's surface that supports the gyroscope) is oscillating about the gyro input axis at frequencies below the first natural frequency of the system, the gyro will be misaligned by an angle sufficient to provide a component of earth's rate equal to the rate at which the erection system is oscillating in order that no rotations with respect to space be sensed by the gyro. The misalignment angle .phi. is given by .phi. = .theta.W/W.sub.e where .theta. is the magnitude of the erection error occuring at frequency W and W.sub.e is earth's rate. Thus the transfer functions between alignment error and erection error (.phi.)/.theta. must have a peak value at least as large as W.sub.N /W.sub.e, where W.sub.N is the first natural frequency of the system. Since settling time T is inversely proportional to the lowest natural frequency, this ratio W.sub.N /W.sub.e could be written K/W.sub.e T. The tradeoff between alignment error due to erection errors occurring around the natural frequency of the system and settling time T then becomes evident. For a system with a nominal settling time of 25 minutes, the ratio W.sub.N /W.sub.e is 20. It should be noted at this point that the settling time T here referred to is the total settling time for the two axis system using decoupling compensation networks to be descriped hereinafter.
The type of gyrocompass to which this invention can be applied may be described as an accurate, azimuth reference with short settling time and preferably would comprise a four-gimbal assembly supporting a limited angle two degree of freedom gyroscope. No particular gyroscope within the foregoing class would constitute a limitation on the execution of this invention. In fact, widely different types of two degree of freedom gyroscopes may be used in a gyrocompass according to this invention. For example, one might use a cryogenic gyroscope as disclosed by Buchhold, U.S. Pat. No. 3,044,309; an electrostatic gyroscope as disclosed by Boltinghouse, U.S. Pat. No. 3,443,320; a free rotor gyroscope, e.g., Autonetics G-6 Free Rotor Gyro; or a tuned rotor gyroscope, e.g., Litton G-2 Gyro also known as "Vibragimbal" or Singer, Kearfott Division, MITA-4 "Gyroflex" (Reg. T.M.). For that reason, the following references to a packaged gyro instrument represent any packaged two degree of freedom gyro having limited angular movement relative to its instrument casing and having signal and torque generators or pick offs for sensing and causing (respectively) angular motion of the gyro rotor with respect to the gyro instrument case. These generators include a signal generator and a torque generator for each of the two input axes which with the gyro rotor spin axis are the three orthogonal axes of the gyro. Angular motion about these two input axes, which may be referred to in either order as a first and second input axis, will cause motion of the spin axis relative to the gyro case. In this sense, the spin axis may be said to have two degrees of freedom (rotational) with respect to the gyro instrument case. In the exercise of this invention any packaged gyro to which the invention is applied must be mounted in a multi-axis gimbal assembly to provide the proper degrees of freedom. My description is based on use of a four gimbal assembly wherein the outer two gimbals of the four gimbal assembly are used only to provide a stabilized horizontal platform using accelerometers or pendulums as a reference although any other conventional method of establishing a stabilized platform could be used. The inner two gimbals of such four gimbal assembly are employed to control rotation of the packaged gyro about two axes of gyro and gimbal assembly which are the azimuth and latitude axes of that assembly. The assembly is connected in a "North seeking" mode wherein the object is to align the gyro rotor spin axis so that it is parallel to the spin axis of the earth. The gyro rotor spin axis is slaved to the gyro case by the torque generators as already noted so that the rotor spin axis is maintained in alignment with a spin reference axis fixed to the gyro case. If the rotor axis is not parallel to the earth spin axis, realignment could be accomplished either by moving the rotor spin axis relative to the case or by moving the case which is followed by the rotor relative to the earth. This invention contemplates doing the latter by moving the case about the inner two multi-axis gimbals of the gimbal assembly. In maintaining the rotor-to-case relationship, the signal and torque generators produce signals representing the alignment error between the gyro rotor spin axis and the earth's rotational axis. The signals produced are two error signals, each reflecting the angular error between the gyro rotor spin axis and the earth's rotational axis as seen about one of the two gyro input axes. These error signals are used to drive the inner two gimbals of said four gimbal assembly until the errors are reduced to zero (nulled). Except for undesired effects, the gyro rotor spin axis is then parallel to the earth's spin axis.
In the present invention only the inner two gimbals of the multi-axis gimbal assembly are considered except for the errors due to tilt of the horizontal reference (i.e., stabilized platform surface). It has been demonstrated that the basic and well known "North seeker" system briefly outlined above has a fundamental limitation in that its initial alignment settling time is in the order of days. It has been determined that this inherent slowness is due to the fact that with gyrocompassing taking place about two cross-coupled axes simultaneously, the gimbal rates being used in correction are indistinguishable from the earth's rates so that the system has to "wait" for the earth's rate components. The present invention makes available, for application to conventional gyro systems, a technique and circuits for compensating the error signals in the process of converting them to gimbal drive signals so as to remove the portion of the signals due to the motion of the gimbals and thereby reduce the settling time of the system so as to provide an accurate, short settling time azimuth reference within a time period of approximately 30 minutes.